Infrared properties of SiC particles

We present basic laboratory infrared data on a large number of SIC particul ate samples, which should be of great Value for the interpretation of the 1 1.3 mu m feature observed in the spectra of carbon-rich stars. The laborato ry spectra show a wide variety of the SiC phonon features in the 10-13 mu m wavelength range, both in peak wavelength and band shape. The main paramet ers determining the band profile are morphological factors as grain size an d shape and, in many cases, impurities in the material. We discovered the i nteresting fact that free charge carriers, generated e.g. by nitrogen dopin g, are a very common characteristics of many SIC particle samples. These fr ee charge carriers produce very strong plasmon absorption in the near and m iddle infrared, which may also heavily influence the 10-13 mu m feature pro file via plasmon-phonon coupling. We also found that there is no systematic dependence of the band profile on the crystal type (alpha- vs. beta-SiC). This is proven both experimentally and by theoretical calculations based on a study of the SiC phonon frequen cies. Further, we give optical constants of amorphous SiC. We discuss the i mplications of the new laboratory results for the interpretation of the spe ctra of carbon stars.